Republican deteriorated and went mad (5)
I believe the answer is:
raged
'went mad' is the definition.
(rage can mean to become angry or mad)
'republican deteriorated' is the wordplay.
'republican' becomes 'R' (political abbreviation).
'deteriorated' becomes 'aged' (ageing is a kind of deteriorating).
'r'+'aged'='RAGED'
'and' is the link.
(Other definitions for raged that I've seen before include "Fumed" , "Was violently angry" , "Spoke loudly and angrily" , "Spoke violently" , "seethed" .)